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• By 1924, approximately 3,000 people had been
  involuntarily sterilized in America the vast
  majority (2,500) in California.

It is better for all the world, if instead of
waiting to execute degenerate offspring for crime
or to let them starve for their imbecility,
society can prevent those who are manifestly
unfit from continuing their kindThree
generations of imbeciles are enough. Justice
Oliver Wendell Holmes, Jr. U.S. Supreme Court
Buck vs. Bell, 1927

• Buck v. Bell supplied a precedent for the
  eventual sterilization of approximately 8,300
  Virginians

• sterilization of people in institutions for the
  mentally ill and mentally retarded continued
  through the mid-1970's. At one time or another,
  33 states had statutes under which more than
  60,000 Americans endured involuntary
  sterilization.

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Announcements

• How is powerpoint slide printing going?
• Bring FlyLab to Lab next week meet in Brooks 101
  (computer lab) for the first part of lab
• Homework this week Ch.2, problems 2, 10, 13,
  14, 19 (NOT turned in)
• Answers to Ch.2 problems will be posted on
  Tuesday, Sept. 3 outside my office
• http//www.eugenicsarchive.org/html/eugenics/essay
  8text.html
• Quiz today!

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Review of last lecture

• Basic concepts that underlie the study of
  genetics DNA, genes, chromosomes
• 2. Somatic cells have a diploid of
  chromosomes (2n)
• each chromosome type (except X and Y) exists as
  a homologous pair
• 3. Different forms of the same gene exist as
  alleles
• ex. wt vs. mutant CFTR gene
• 4. How do scientists investigate genetics?
• 5. Genetics and society - eugenics,
  agriculture, medicine
• 6. Mitosis is one part of the cell cycle
  important for many reasons
• 7. Four phases of mitosis prophase, metaphase,
  anaphase, telophase

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Outline of Lecture 3

• I. Cell division is genetically regulated
• II. Meiosis
• III. Gregor Mendel - discovered basis for
  transmission of hereditary traits
• IV. Monohybrid cross
• V. Mendels postulates

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I. Cell division is genetically regulated

• Why are we interested in knowing how cell
  division is regulated?
• if regulation is disrupted, uncontrolled cell
  division may
• result..cancer
• Most recent Nobel Prize was awarded to 3
  scientists who studied
• genes that regulate the cell cycle, including
  Lee
• Hartwell (director of the Fred Hutchinson Cancer
  Research
• Center) who studied cell division regulation in
  yeast
• http//www.fhcrc.org/visitor/nobel/hartwell/accom
  plishments.html
• There are 3 main checkpoints in the cell cycle

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Three main checkpoints in the cell cycle

• Is cell the correct size?
• Is DNA damaged?
• 2. Is DNA fully replicated?
• Is DNA damage repaired?
• 3. Have spindle fibers formed?
• Have they attached to
• chromosomes correctly?

1.
3.
2.
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Why are cell cycle checkpoints important?
What might result if DNA repair has not finished?
Uncontrolled cell division could occur -
cancerous cell
Example p53 protein normally targets cells with
severe DNA damage to undergo programmed cell
death. (this removes them from the
population) If the p53 gene is mutated, damaged
cells will not be removed and may continue
dividing in an uncontrolled manner. Many
different types of cancers involve mutations of
p53.
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II. Meiosis
a special cell division to make gametes (sperm
and egg)
Why would a regular mitosis be a problem in
making gametes?
If

then
4n
2n
2n
sperm,
egg
embryo

• Meiotic cell division generates cells (sperm and
  eggs) with one- half the genetic material (2n to
  1n) - a reduction in chromosome number
• Source of genetic variation - see mechanics

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Key points of meiosis

• Homologous chromosomes pair (synapse) to form a
  bivalent the four chromosomes form a tetrad.
• Recombination during meiosis is the basis for
  genetic variation within species.
• Two divisions reductional division and
  equational division, each with four phases

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Mitosis vs. Meiosis

• S phase 2N ? replication ? duplicated 2N
• Mitosis duplicated 2N ? separation of sister
  chromatids ? each daughter cell is 2N
• Meiosis duplicated 2N ?
• meiosis I (reduction division) separation of
  homologous chromosomes ? synapsis of homologous
  chromosomes ? recombination duplicated N
• meiosis II (equational division) duplicated N ?
  separation of sister chromatids ? N
• Is Meiosis I or II more like mitosis?

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Meiotic Prophase I(5 stages of prophase I)

• 1. Leptonema slender-thread
• Condensation chromatin starts to condense
• 2. Zygonema paired-thread
• Pairing homologues pair (synapsis) in
  synaptonemal complex (not in mitosis)
• s.c. allows for crossing over if it doesnt
  form, no synapsis, no crossing over

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Meiotic Prophase I (continued)

• 3. Pachynema thick-thread
• Ea. tetrad has 2 pr. sister chromatids
• Recombination further condensation crossing
  over occurs
• 4. Diplonema doubled-thread
• tetrads visible, chiasmata visible (where sister
  chromatids contact)
• 5. Diakinesis movement apart
• Breakaway sister chromatids pull apart,
  chiasmata move to ends of each tetrad
• NEBD, nucleolus disappears, spindle fibers attach
  to centromeres

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Completion of Meiosis I

• Metaphase I
• tetrads align randomly independent assortment
• Anaphase I
• one-half of each tetrad, a dyad (homologue),
  moves to each pole
• sister chromatids together
• separation of tetrads is disjunction when they
  do not separate it is nondisjunction - more ch.
  10
• Telophase I

Met I
Ana I
Tel I
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Meiosis II

• Mechanistically similar to mitosis.
• Sister chromatids separate, producing monads.
• Four haploid gametes can potentially form.
• If crossing over occurred, ea. monad has combined
  genetic information

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GametogenesisSpermatogenesis

• Occurs after puberty, continuously in human
  males.
• Equally-sized haploid products sperm
• Crossing over can occur to create genetic
  recombination.

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Gametogenesis Oogenesis

• Begins during first months of embryogenesis in
  human females.
• Meiosis arrests at diplotene of prophase I and
  resumes after puberty at ovulation.
• Unequally-sized haploid products huge egg and
  tiny polar bodies.
• Meiosis arrests again at metaphase II and resumes
  after fertilization.

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Multiple Choice - self test

• Which of the following is true about cell
  division
• Meiosis I is more like mitosis because it is a
  reductional division (2n to 1n)
• Meiosis I is more like mitosis because sister
  chromatids separate
• Meiosis II is more like mitosis because it is an
  equational division (1n to 1n)
• Meiosis is similar to mitosis because it
  generates genetic variation

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III. Gregor Mendel

• Monastery of St. Thomas, Brno, Czech Republic.
• Taught physics and natural science.
• Performed experiments 1856-1868, published in
  1866.
• Why peas?
• Easy to grow
• Self-fertilize or can hybridize artificially
• Matures in single season
• Choice of contrasting traits

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How Mendel performed his crosses with pea plants
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Mendels 7 traits
1. 2. 3.
4.
5. 6.
7.
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Modern genetic terminology

• Phenotype - physical expression of a trait
• Gene - Mendels unit factors of inheritance
• Allele - different forms of a gene, e.g. D or d
• Genotype - allelic composition of a trait
• e.g. DD, Dd, or dd

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More modern genetic terminology

• Homozygous - genotype of identical alleles, e.g.
  DD or dd
• Homozygote - homozygous individual
• Heterozygous - genotype of different alleles,
  e.g. Dd
• Heterozygote - heterozygous individual
• Dominant and recessive - Alternative phenotypes
  when two alleles are expressed.
• D is dominant and d is recessive if Dd and DD
  have the same phenotype

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IV. Monohybrid cross (tall and dwarf pea plants)
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Monohybrid Cross Punnett Square Method
(1) Define symbols D tall allele d dwarf
allele (2) State the cross (3) Diagram the
gametes (4) Complete the squares (5) Summarize
the results Genotype Phenotype
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Reciprocal crosses

• Results were the same regardless of which parent
  was used, e.g.
• tall pollen pollinating dwarf eggs
• dwarf pollen pollinating tall eggs
• Therefore the results were not sex-dependent
• Mendel proposed unit factors to explain his
  results

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V. Mendels postulates
Postulate 1. Unit factors in pairs

• Genetic characters are controlled by unit factors
  in pairs.
• In other words, genes are present in two
  associated copies in diploid organisms.
• For example, DD plants have two alleles for
  tallness, dd plants have two alleles for
  dwarfism.

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Postulate 2. Dominance/recessiveness

• In the case of unlike unit factors, one can be
  dominant and the other can be recessive.
• In other words, when two different alleles of a
  gene are present, one may show its effect while
  the other may be masked.
• For example, Dd plants have a tall allele D and a
  dwarf allele d, but are phenotypically tall.

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Postulate 3. Segregation

• During the formation of gametes, unit factors
  segregate randomly.
• In other words, when sperm and eggs are formed,
  one of each allelic pair is randomly distributed
  to to each gamete.
• For example, a Dd plant makes pollen or eggs,
  each randomly receives either the D allele or the
  d allele.

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Practice Axial/Terminal Pods

• In garden peas, an allele T for axial flowers is
  dominant to an allele t for terminal flowers.
• In the F2 generation of a monohybrid cross, what
  is the expected ratio of axial terminal?
• Among the F2 progeny, what proportion are
  heterozygous?
• Among the F2 progeny with axial flowers, what
  proportion are heterozygous?

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Round/Wrinkled Seeds

• Round (W) dominant to wrinkled (w)
• Whats the molecular basis? Starch and sugar
  content
• Wrinkled seeds have higher glucose, water content
  before drying, larger loss of seed volume during
  drying

Loss-of-function in wrinkled allele